Introduction

We now understand how fields propagate in an unbounded medium as plane waves and carry energy from one point to the other, how energy is transmitted along a transmission line, and how energy is channeled through a waveguide. We will now discuss systems that not only generate electromagnetic fields but also radiate them effectively.

Maxwell's equations dictate that in order to create electromagnetic fields we require time-varying sources such as charges and currents. When the fields created by these sources are confined to propagate as a wave along a transmission line or inside a waveguide, the wave is usually referred to as a guided wave. When these sources, finite in size, create waves that propagate away from them in an unbounded medium, they collectively form a radiating system, and the process is called the radiation of electromagnetic waves. The device at the end of a radiating system is referred to as a transmitting antenna (see Figure 11.1). A transmitting antenna, when fed by a transmission line as shown in Figure 11.2, is called a dipole antenna. Figure 11.3 shows a horn antenna fed by a waveguide. Among other types of antennas are a slot antenna (a slot in a large conducting plane fed by a waveguide), Figure 11.4, and a microstrip antenna (a thin patch of metal on a grounded dielectric substrate), Figure 11.5. When an antenna is used to capture the radiated energy, it is called a receiving antenna.